1. Field of the Invention
The present invention relates to an organic field effect transistor (called “organic FET” hereinafter) using an organic semiconductor material. More specifically, the invention relates to an organic thin film transistor (called “organic TFT” hereinafter) using a thin film containing an organic semiconductor material.
2. Description of the Related Art
In various semiconductor apparatus incorporating semiconductor elements, such as a television receiver, a personal computer and a mobile telephone, a display for displaying text and/or images is required by human beings for recognizing information. Recently, flat panel displays have been positively used such as a liquid crystal display using an electrooptic characteristic of liquid crystal and an organic EL display using electric field emitted light of an organic material.
As one form of the flat panel display, an active matrix driving method has been known in which a thin film transistor (TFT) is provided for each pixel and data signals are sequentially written for video display. A TFT may be an element required for implementing the active matrix driving method.
It is important to note that most of field effect transistors (FET) such as the TFT have been produced by using inorganic semiconductor materials such as amorphous silicon and crystalline silicon. However, when the FET is formed by using these inorganic semiconductor materials, the substrate processing temperature in the process for producing a semiconductor layer, for example, exceeds 350° C. As a result, many of useful substrate materials (such as plastic) cannot be used. This is a disadvantage.
When an FET is produced by using current inorganic semiconductor materials, the insulating layer and semiconductor layer and the electrode are formed by using a plasma CVD apparatus and a sputtering apparatus, respectively. However, these CVD apparatus and sputtering apparatus are relatively expensive and need time for the maintenance.
On the other hand, a method has been proposed for producing an FET by using organic semiconductor materials. An organic compound itself dose not have carriers and has a good insulating characteristic essentially. However, current can be conducted through the organic compound by using a series of materials called organic semiconductor materials (or a conjugate organic compound, in general).
For example, like a conductive high polymer, acceptors (electron acceptors) or donors (electron donors) are doped to the π conjugate organic compound such that the π conjugate organic compound can have carriers due to impurities. Therefore, the conductivity can be caused (see Document 1: “CHEMISTRY COMMUNICATION”, Vol. 16, pgs. 578-580, Hideki SHIRAKAWA et al, 1977). By increasing the doped amount, the amount of carriers increases to a certain range. Therefore, the dark conductivity increases therefor, and a more amount of current can flow threthrough.
In this way, the measure for increasing the dark conductivity by doping impurities (such as acceptors and donors) in order to conduct current through an organic semiconductor material has been partially applied in the electronics field. The examples include a chargeable secondary battery using polyaniline and/or polyacene and a field effect capacitor using polypyrrole.
By applying such a measure for an organic semiconductor material, an organic FET can operate. The organic FET has basically the same construction as a conventional FET using an inorganic semiconductor material except that an organic semiconductor material is used as an active layer instead of an inorganic semiconductor material (the active layer containing an organic semiconductor material is called “organic semiconductor film” hereinafter). Many reports have been made on this kind of organic FET.
For example, an organic TFT has been reported using, as an active layer, poly (2, 5-polythienylenevinylene) (called “PTV” hereinafter), which is a kind of conductive high polymer material. The report is provided in Document 2, “Applied Physics Letters”, Vol. 63, pgs. 1372-1374, H. Fuchigami et al., 1993. The PTV described in Document 2 is doped to p-type by oxygen in the air, which is weak acceptor and exposes moderate conductivity as an active layer. In this way, the conductivity of conductive high polymer materials can be controlled easily by doping. Therefore, conductive high polymer materials gathers attentions as materials to be used in an organic FET.
In Document 2, moderate current may flow between the source and the drain by applying the method for causing carriers in an organic semiconductor film by doping impurities. The organic FET is called “dope-type organic FET” hereinafter.
On the other hand, for example, an organic FET may be produced by using a fine crystalline thin film containing a low polymer organic semiconductor material without doping an impurity to an organic semiconductor film. This example is described in Document 3: “IEEE Electron Device Letters”, Vol. 18, pgs. 87 to 89, D.J. Gandranch et al, 1997. In Document 3, a silicon oxide film is formed on a high-dope silicon wafer as an insulating film. Then, fine crystal of pentacene, that is an organic semiconductor material, is stacked thereon by vacuum evaporation. Furthermore, a source electrode and drain electrode of gold are formed thereon. As a result, an organic FET can be obtained.
In Document 3, carriers due to impurities do not exist in the organic semiconductor film. However, since a fine crystal thin film with higher mobility is used, carriers injected from the source and drain can move. Therefore, the result can operate as an FET, which is called “carrier-injected organic FET” hereinafter.
These kinds of organic FET can be formed at a lower substrate temperature. Therefore, a flexible substrate of plastic, for example, can be applied. Furthermore, the organic FET other than a monocrystal thin film and a fine crystal thin film can be formed easily by general vapor deposition or spin coating. Therefore, the production process can be simplified and can save energy. Because of these advantages, an organic FET is gathering attentions as a new FET instead of an FET containing an inorganic semiconductor material.
However, a conventional organic FET as described above has problems due to impurities (acceptors and/or donors) or the characteristics of the organic semiconductor film itself.
First of all, as described in Document 2, when acceptors and/or donors are doped to an organic semiconductor material so as to cause carriers therein (that is, in the case of the dope-type organic FET), the acceptors and/or donors themselves are not chemically stable, and the production is not easy. For example, alkali metal and alkali earth metal can be used as the donors. However, the donors themselves are extremely active to water, oxygen and so on. Therefore, the use may be difficult.
Furthermore, when an impurity is doped to an organic semiconductor material, electrons are exchanged between the organic semiconductor material and the impurity, that is, a kind of chemical reaction occurs. Therefore, the doped condition itself may not be stable.
On the other hand, in the case of a carrier-injected organic FET as disclosed in Document 3, the organic semiconductor material does not need to have carriers. Therefore, problems relating to unstableness due to acceptors and/or donors do not occur. However, in order for injected carriers to flow between the source and the drain, an organic semiconductor film having higher mobility than conventional one is required in consideration of the distance between the current source and drain. Therefore, monocrystal or fine crystal must be used presently. A monocrystal thin film and fine crystal thin film containing an organic semiconductor material are difficult to produce and are not realistic. This is another disadvantage.
Furthermore, when it is assumed that a monocrystal thin film and a fine crystal thin film can be formed, a high polymer material is basically difficult to use. Thus, forming a thin film by wet coating-is not easy. This is another disadvantage. Because of these disadvantages, the number of material selections are limited for a carrier injected type organic FET.
In view of these disadvantages, an organic semiconductor film produced by using an extremely common vapor deposition film and/or coated film is desirably used to operate an FET without adding acceptors and donors and without using monocrystal and fine crystal.
For example, an attempt has been made in which carriers are caused by optically exciting an organic semiconductor film and by feeding current due to the carriers between the source and the drain. Thus, an FET can be operated. The attempt is described in Document 4: “Applied Physics Letters”, Vol. 79, No. 12. pgs. 1891-1893, K.S. Narayan et al., 2001.
The improvement in operational function as an FET is proved where carriers can be caused in an organic semiconductor film by light irradiation. However, in the method disclosed in Document 4, the step of the moderate light irradiation is further required for the FET operation. Therefore, in order to operate the FET only electrically, the method is not suitable and is not a realistic solution.